1. Field of the Invention
The present invention relates to the field of correction of colors of photographic images. The present invention relates in particular to a color correction method, to a program, which performs the color correction method, to a program storage medium on which the program is stored, to a photographic image processing device, which performs such a correction, to a photographic printer, to a photographic laboratory and to a display device for displaying photographic images (e.g. monitor and computer or only monitor, like LCD-display, CRT-display) in which such a correction method is implemented.
2. Description of the Related Art
Photographic images are recorded by means of photographic image capturing devices like cameras (still cameras, moved picture cameras, video cameras, digital cameras, film cameras, camcorders, etc.). The picture information of photographic information carried by light is captured by the cameras and recorded e.g. by means of a semiconductor chip or photochemically on a film. The analogue recorded image information is then digitalized e.g. by means of a analogue-digital converter or by scanning a film in order to achieve digital image data. The digital image data are then processed in order to transform the data in a status in which they are suitable for being displayed to a user by means of an output device (e.g. printer plus print medium or monitor).
Starting from the situation of capture of the photographic image until the final display of the image for the user or the storage of the image data for a later display, there are a lot of possible error sources, which may affect the photographic image data such that the image displayed to the user deviates from the optical appearance of the photographic object to a human being at the time the picture has been taken. The present invention relates to deviations in color.
The origins for such kind of errors or deviations may be of technical nature as well as may have their origin in the way the human being perceives colors and images. Technical causes may be, for instance, chromatic aberration of the lens system, color balance or white balance algorithms in digital cameras, spectral sensitivity of CCD chips or films, the application of color correction algorithms etc. The colors of a photographic object captured by a camera, of course, depend on the illumination spectrum. In contrary to this the human color correction system has a so-called “color constancy” feature. The human being is able to identify color samples of different color values even under different illumination conditions based on his memory about the color value (see “Measurement of Color Constancy by Color Memory Matching”, Optical Review, Vol. 5, No. 1 (1998), 59-63, respectively http://www.JSST.OR.JP/OSJ-AP/OpticalReview/TOC-lists/vol05/5a059tx.htm.) The color constancy is a perceptual mechanism, which provides humans with color vision, which is relatively independent of the spectral content of the illumination of a scene. In contrary to this, the color values recorded by cameras only depend on the spectrum of the illumination light (e.g. tungsten light, flash light, sun light).
Additionally the human being has a good memory for colors of objects, which he often encounters in daily life, like the color of skin, foliage, blue sky, neutral or grey (e.g. the color of streets is grey). For instance, the skin tone is rather sensitive to slight color tone changes. For instance, in the CMYK (cyan, magenta, yellow, and black) color space the relationship for a Caucasian (European) skin tone is 13C-40M-45Y-0K. This applies at least for young women and children. Typically magenta and yellow is close to equal and cyan is about ⅓ to ⅕ below magenta and yellow. If magenta is higher than yellow, the skin tone will look red. If yellow is much higher than magenta, the skin tone will look yellow. Black should be only in shadow areas of the skin tone or on darker skin tones (see, for instance, http://www.colorbalance.com/html/memory_.html).
Since those kinds of memory colors are often present in photographic images, they represent characteristic colors for photographic images and may be used as a reference for color correction.